Wi-Fi is a wireless radio technology that allows electronic devices to exchange data. As defined by the IEEE 802.11 specification, Wi-Fi is a polite protocol that operates on the principle of “listen before talk.” A station (STA) implementing one of the 802.11 standards uses a Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) mechanism to sense other transmissions and backs off from transmitting until the channel is sensed as “idle.” According to the rules of CSMA/CA and the 802.11 Distributed Coordination Function (DCF), a STA that wishes to transmit must first listen on the channel for a period of time defined by the DCF Interframe Space (DIFS). If the channel is sensed as “idle” for the duration of the DIFS time period, the STA assumes that no other station is transmitting and that it can begin transmission. If, however, the channel is sensed as “busy”, the STA must back off and wait for a random back-off interval before trying to transmit again. In addition, after each transmission, the STA must wait for a random back off period before transmitting again.
A problem with detecting that another station is transmitting is that Wi-Fi stations are not able to transmit and receive at the same time. This makes it difficult for a STA to determine that a collision is occurring or has occurred. To compensate, 802.11 STAs rely on acknowledgement (ACK) frames to indicate that a transmission sent by the STA occurred without error. If the STA has not received an ACK frame within a given timeout period, it knows the transmission has failed for some reason and that the failure may have been due to a collision. The STA therefore, on timing out while waiting for an ACK, backs off for a random back-off interval before trying the transmission again.
Another problem with detecting that another station is transmitting is that a Wi-Fi station may not be in radio range of the station causing the collision. For instance, a station (STA) A may be ready to transmit to an access point (AP) at the same time that STA B is transmitting to the same AP. When STA A can hear the AP but cannot hear STA B. STA B is considered a “hidden node.” The Request to Send/Clear to Send (RTS/CTS) frame exchange addresses the “hidden node” problem by having STA B transmit an RTS frame and wait for the AP to transmit a CTS frame in response. Even if STA A cannot hear the RTS from STA B, it should be able to hear the CTS frame sent in response by the AP. Each CTS frame includes a time value used by the other stations to hold off transmission while STA B transmits. This approach is, however, seldom used since it increases the overhead of data transmission.